A typical method of securing PV modules to roofs using a wood deck is with a rack system including vertical stanchions and lateral rails. The vertical stanchions are often lag bolted into joists, which are typically on 24″ (61 cm) centers. Conical flashings, similar to the type used for ventilation pipes, are used to waterproof these penetrations. In some cases flashings are not used and “L” brackets or other mounting hardware is lag bolted directly through the roofing material, with the penetration caulked with sealant. Then lateral rails are attached to the stanchions, typically several inches off the roof to allow clearance for the flashings. PV modules are then attached to the rails. Reasons for using the vertical stanchions and a lateral rails approach include: PV modules are not typically designed in convenient widths relative to joist spacing, not all PV modules have geometries amenable to direct-deck mounting, and the racks are designed to accommodate generally any PV module. In most cases framed PV modules are mounted in this manner but methods to mount unframed PV modules to racks do exist.
In another method for securing PV modules to roofs, the PV modules are typically lag bolted into blocking members installed between rafters in the attic; other mounting hardware can also be used. Relatively large holes must often be pre-drilled through the roofing material to accommodate the mounting hardware. Because of the size of these larger holes and the configuration of the module, it is often difficult to tell if adequate waterproofing has been achieved. If blocking is used, the process of installing blocking involves extensive work in the attic which adds significantly to installation time.
A further method for securing PV modules to roofs uses a hold down device that can only be used with specially constructed PV modules having complementary hold down structure, such as laterally extending hold down pins.